1. Field of the Invention
This invention relates broadly to electronic security systems. More particularly, this invention relates to an acoustic intrusion detection system utilizing audible stationary sound waves.
2. State of the Art
Electric or electronic security systems have been in use for nearly 100 years. These systems employ many different kinds of sensors to detect an unlawful intrusion into a protected space. One such sensor is a motion detector. The most popular motion detectors are infrared (IR) and ultrasonic. Despite the many advances in the sophistication of security systems, some intrusions go undetected. At other times a sensor produces a false positive detection.
While the problem of an undetected intrusion is self-evident, false positives also pose a significant problem. Typically, when an intrusion is detected, a signal is sent to a central monitoring station which monitors the security systems of many customers. The monitoring station then informs the local police to investigate the intrusion. Most police departments have a policy that if they are called more than a certain number of times for a false positive intrusion detection, they will not respond to any more calls regarding that site.
False positive detection by ultrasonic motion detectors can be triggered by many different events including wind, loud noises near the protected space, and the movement of rodents or other small animals. In order to minimize false detection, some security companies install listening equipment in the protected space. When a motion detector triggers an alarm, someone at the monitoring station listens to hear if there is real intrusion or a false alarm. This works sometimes, but not always, and requires human resources.
State of the art motion detectors need a “line of sight” to the moving object to detect the motion. Because of this limitation, it may be necessary to install several motion detectors in the protected space.
Many known motion detectors are also adversely affected by change in temperature. Many also require a relatively fast digital signal processor. IR motion detectors are easily disabled with hair spray. Ultrasonic detectors can be disabled by covering them with a sound absorbing cover. It should be noted that many intrusions are by employees who attempt to disable the security system during the day so they can return at night undetected.